Controlling Multi-input Converters to Act as Electric Energy Router

Abstract: It is undeniable that the energy consumption will continue to grow and the negative impact of using fossil fuels to produce it will favor the use of alternative energies. There has been an intensive research to develop clean energies from some decades now. As a result electric energy can now be generated using photovoltaic cells, wind power, fuel cells among other clean sources. Each source has advantages and disadvantages, hence, it is generally accepted that a combination of them is the best. Power electronics specialists have started to develop schemes to allow sources of different characteristics to cooperate to power a load. The device that achieve this goal has been called “energy router” by some researchers. In this work, it is shown that common multi-input power converter can function as energy routers if they are properly controlled. A control algorithm is proposed for these converters

It is expected that in 2050 the world population will rise to 9 billion from its current level of 7 billion. At the same time it is projected that the world economy will grow almost four-fold, causing an increase of energy and natural resources demand [13]. There are various measures today that are being implemented to reduce the fossil energies and electric energy trying to assure a continuous and sustainable energy input for the future, and reaching the world CO2 emission reduction goals in 2050 to avoid a climatic chaos.

It is expected that the electric energy demand will grow 85% between 2009 and 2035 to meet the CO2 emission goals at least 44% of this energy will must to be generated from renewable sources. Renewable energies can be integrated in all kind of electric systems, from huge interconnected networks to small autonomous systems or buildings.

Some of the problems presented by renewable energies come from the fact that they depend on changing and uncertain environmental conditions, the load variability and perturbations present in the distribution network when they are interconnected to it (see [7, 19]). One solution to maintain a constant energy supply are the Hybrid Energy Systems, that can combine one or more renewable energy sources with traditional storing systems that can also be interconnected with the electric energy infrastructure.

The goal of an Hybrid Energy System is to supply the demanded energy using a renewable source, even when, by itself, it’s not capable of sustaining the demand. This hybrid system stores energy when the renewable source exceeds its demand, and when the demand grows it takes energy from here. If the storage system is at full capacity, and the demand is less than this, there’s the possibility of pumping the excess into the distribution network. It can also take some energy from the distribution network when the hybrid system is not enough to satisfy the demand. Other realizable function is to integrate and interconnect various renewable sources, like photovoltaic or bio-mass production, for example [9, 12, 15].

The interconnection of different renewable energy sources in the Hybrid Energy Systems, is a complex subject, since voltage levels can vary widely [9, 12, 15]. For example, the interconnection of an eolian generator and a photovoltaic one, taking into account multiple climatic conditions, such as a sunny day with no wind, or a cloudy day with a lot of wind and very little luminosity. So a device which allow to modify the energy proportion supplied by each source is required to take advantage from the renewable energies. So, to improve the usage of renewable energies, it is necessary to have a device capable of manipulating the power from every source, working cooperatively, and not one that just maintains a constant voltage or current. Such a device has been called energy router [17, 22].

The first ideas to develop energy routers came from the so called Duindam-Stramingioli energy router. This is a mechanical device that allows several mechanical power sources to cooperate to move a body. Based on these ideas a topology of power converter to transfer energy between several devices has been proposed recently. However, till now there are not practical examples of electronic energy routers that allow several sources to feed a load cooperatively.

Ideally, an energy router should have the capacity to decide the energy percentage from every source in relation to the energy produced, cost and demand. In [17] Sánchez et al present a subsystem called Dynamic Energy Router that dynamically controls the energy flux from every source with a parameter. They show the concept using a circuit with 2 super capacitors and prove that the energy transfer exists from one super capacitor to another with a parameter they denominate α.

Although this device transfers the energy from one point to another, it just allows the connection of two elements and the transfer of energy between them. In practice is common to find systems that require the connection of generating and consuming elements with very different dynamics. A topology that can be used as an energy router and allows to interconnect at least two different power sources is difficult to find. The topologies proposed by [17] allows to perform the energy routing, but can not connect two sources to feed a load. An improvement to this topology is presented in [22], proving that is possible to maintain the routing capability interconnecting a battery and two super-capacitors by two different topologies. The experimental results presented demonstrate the routing capability, but to implement these topologies for fuel cell applications an extensive research should be developed.

In recent years derived from DC-DC converters, multi-input converters have been developed, allowing to feed a load by at least two different voltage DC sources [1, 4, 5, 18]. Also to feed an AC load with different sources the matrix converters have been developed [3]. Multi-input converters have been used in different applications such as hybrid vehicles. For example in [25, 26] multi-input converters allow to interconnect different sources to achieve a desired voltage in a DC bus. On hybrid vehicles applications, a fuel cell, a battery and super-capacitors are connected to supply the required energy to the vehicle. Although the sources supply the needed energy to the vehicle, there is no policy to achieve an efficient energy usage.

Other examples to interconnect renewable energies by multi-input converters are presented in [21, 23, 27], in these schemes the main goal is also to regulate a DC bus. Even when these solutions allow the interconnection of two sources working cooperatively the energy is not used efficiently. Most cases it is stored in batteries or sent to the electrical network. From the previous references it can be observed that control of multi-input converters has focused in regulating the load voltage [4, 5, 16], regardless of the optimal way of using the energy.

In this paper using the equilibrium analysis it is shown that the multi-input buck and multi-input boost converters are able to function as energy routers if they are properly controlled. Furthermore a control algorithm for each both multi-input converters are proposed.

In Section 2 our view of the requirements for a circuit to be called an energy router is precised. Based on this understanding, in Section 3 a way to enable the multi-input buck converter to function as an energy router is presented. In Subsection 3.1 its model is obtained. Using an equilibrium analysis, in Subsection 3.2 it is shown that this converter could be made to work as an energy router. Then in Subsection 3.3 a control to actually make this converter to function as an energy router is presented. Simulations results of the control algorithm proposed is presented in 3.4. In Section 4 it is shown that the multi-input boost converter can also be enable to function as an energy router. Finally in Section 5 some conclusions are made.